To cap a 120,000sq.m lagoon containing ultra-soft sludge at Knostrop Waste Water Treatment Works (WWTW) adjacent to the A1M near Leeds, Yorkshire Water turned to Mott MacDonald Bentley (MMB) to devise a bespoke solution. MMB based their solution on experience gained from an earlier scheme on a smaller sludge lagoon, at Dowley Gap near Bingley in Yorkshire. In this project, MMB collaborated with engineers from Tensar International to identify the optimum method utilising Tensar Technology to stabilise the capping over the sludge lagoon, enabling it to be left as a grassed site.
The fluid nature of the waste deposits at both Dowley and Knostrop means that the working methods need to minimise displacement, settlement and heave of the contents without allowing them to become uncontrollable. If this happened, the situation would be difficult to recover and could jeopardise continuation of the capping technique.
A system was devised to establish initial safe access using access roads at about 30 metre intervals to construct a bearing layer and enable conventional plant and aggregate delivery wagons to move safely about the site and complete the earthworks and landscaping. The methodology for placing the fill is critical to avoid displacement of the sludge and Tensar engineers were able to advise MMB on appropriate methods based on their experience from similar lagoon capping projects in Europe.
At an early stage of the work, MMB conducted a number of site trials to advance the technique by investigating variations to the installation technique and examining the scope for further economies. The variables were the number of geogrid layers, the fill layer thickness and alternative types of geogrid.
The outcome was that MMB maintained their confidence in Tensar geogrids and selected two varieties of Tensar biaxial geogrids and Tensar SSG geocomposite to mechanically stabilise the fill layer. This combination allows water to pass but prevents silts rising through the geogrid reinforcement.
Further capping progress
Following the stabilisation process, a capping layer is being added in phases, with Tensar biaxial geogrids interleaved with recycled aggregate from around Yorkshire. As the work proceeds, Tensar is maintaining contact with the project and offering assistance on refining the techniques – such as the introduction of a level monitoring scheme to provide data on the behaviour of the mobile sludge, (mainly the heave and wave effects), and indicate where it would be prudent to place further fill in order to keep in control of the situation.
Following this, MMB will install a geocomposite gas dispersal layer, a 1mm capping membrane to seal the sludge from further water ingress and a 6mm geocomposite drainage and protection layer. Surface water is channelled to perimeter drains which run around the edge of the lagoons.
Predicted methane levels are low, to be monitored at regular intervals to check levels of the methane let off. To finish, 850mm of subsoil and 150mm of topsoil will be added to enable its return to a more natural, landscaped appearance, completion is due in spring 2008.